This book, the sixth in the series, follows the adventures of Rick and Scotty as they travel to the Pacific Ocean with some other Spindirift scientists to map a part of the ocean. While out in the Pacific they become entangled with the fabled Phantom Shark, a vicious criminal who steals priceless pearls from divers and sells them on the black market. This Shark is bent on stopping Spindrift's mapping expedition and Rick and Scotty must find out why - and discover the real identity of the Phantom Shark.